Using the mutant of Saccharomyces cerevisiae which requires unsaturated fatty acid for growth, we propose to study the effect of varying the unsaturated fatty acid composition of the mitochondrial membrane on the synthesis and assembly of the peptides of cytochrome oxidase and oligomycin sensitive ATPase. Furthermore we will study the intracellular locus and physiological properties of some of the enzymes of phospholipid biosynthesis, especially those involved in cardiolipin biosynthesis. This will include attempts at the isolation of yeast plasma membrane and rough endoplasmic reticulum in a high state of purity and good yield, to facilitate studies on the exchange of phospholipid between these membranes and those of the mitochondria. We will attempt to demonstrate the exchange of cardiolipin from yeast mitochondria in various physiological states. Phospholipid exchange proteins will be sought in the yeast cytosol.